Implement "updated_since" filter

Harvest allows passing an "updated_since" parameter for reporting API
calls that only return time entries updated after the passed UTC
date/time. This feature is necessary if trying to synchronize a local
datastore with Harvest time data.
Merge branch 'add_updated_since'

* add_updated_since:
  Implement "updated_since" filter
Merge pull request #57 from petemcw/master

Implemented "updated_since" filter for reports